Re: [PATCH 1/3] Separate IRQ-stacks from 4K-stacks option

[Lee Revell]

On Fri, 2004-09-10 at 11:34, Arjan van de Ven wrote:
> On Fri, Sep 10, 2004 at 05:28:52PM +0200, Andrea Arcangeli wrote:
> > On Fri, Sep 10, 2004 at 05:15:38PM +0200, Arjan van de Ven wrote: 
> > > What we should consider regardless is disable the nesting of irqs for
> > > performance reasons but that's an independent matter
> > 
> > disabling nesting completely sounds a bit too aggressive, but limiting
> > the nesting is probably a good idea.
> 
> disabling is actually not a bad idea; hard irq handlers run for a very short
> time

The glaring exception is the IDE io completion, which can run for 2000+
usec even with a modern chipset and drive.  Here's a 600 usec trace:

http://krustophenia.net/testresults.php?dataset=2.6.8-rc4-bk3-O7#/var/www/2.6.8-rc4-bk3-O7/ide_irq_latency_trace.txt

The timer, RTC, and soundcard interrupts (among others) will not like
being delayed this long.  Ingo mentioned that this was not always done
in hardirq context; presumaby the I/O completion was done in a softirq
like SCSI.  What was the motivation for moving such a long code path
into the hard irq handler?

Lee

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